1. Field of the invention
The present invention relates to a lens device comprising a lens holding barrel and a lens element held in the lens holding barrel.
2. Description of Related Art
In recent years, because of a strong demand for a high optical performance and a decreased number of lens elements of a projection lens system for a liquid crystal projector and mass productivity of a lens element even having a complicated shape, it is usual to employ aspherical plastic lens elements made by injection molding. The problem encountered by such a plastic lens element is that, in the case where a clearance between a lens barrel and the plastic lens element is too small, the plastic lens element fails to be snugly installed in the lens barrel or causes deterioration in lens performance due to distortion even if it is forced in the lens barrel. On the other hand, in the case where the plastic lens element is large in diameter than the lens barrel, the plastic lens element is unstable, more specifically turns, in the lens barrel unless secured to the lens barrel with, for example, an adhesive. Generally, even when a glass lens element turns in the lens barrel, there are no problems unless the optical axis is out of alignment. However, when the plastic lens element of a liquid crystal projector lens system turns in the lens barrel, the potential problem is that the plastic lens element has an adverse effect on its optical performance such as an occurrence of distortion of a projected image even if the optical axis remains aligned regardless of turning of the plastic lens element in the lens barrel. This is because the injection molding is difficult to form a plastic lens element with a sufficient precision of shape at a location close to a gate of the mold, leading to formation of local irregularities in shape which gives birth to the distortion.
There have been known various structures for position adjustment of a lens element with respect to an axis of a lens system such as described in, for example, Japanese Unexamined Patent Publication Nos. 2001-209118 and 2005-99116. According to the lens position adjusting structure described in Japanese Unexamined Patent Publication No. 2001-209118, a plastic lens comprises a lens portion and mechanical elements such as positioning racks, adjusting slide portions and a handgrip for adjusting work which are formed as integral parts of the lens portion and form parts of the lens position adjusting mechanism. This lens position adjusting structure is simplified in structure and enabled to perform lens position adjusting work without using a tool. According to the lens position adjusting structure described in connection with a projection lens device which comprises a lens barrel and a projection lens system arranged in an optical path defined in the lens barrel in Japanese Unexamined Patent Publication No. 2005-99116, at least one lens element of the projection lens system which is provided with an annular flange formed as an integral part of a lens portion thereof is adjusted in position three-dimensionally through an access hole formed in a peripheral wall of the lens barrel and fixed within the lens barrel with an adhesion injected through an injection hole formed in the peripheral wall of the lens barrel. This lens position adjusting structure enables precise positioning of the lens element and easy fixation of the lens element in the lens barrel.
However, the problem encountered by the prior art lens position adjusting structures is that the lend element has to have a unique shape and is adjusted in position in troublesome procedure and is poor in versatility and is unfavorable in terms of cost in consequence. More specifically, the first lens position adjusting structure is incompatible with a lens element having a general round shape. On the other hand, the second lens position adjusting structure is not always capable of preventing the lens element from turning after position adjustment unless the lens barrel catches hold of the lens element.